The present invention relates generally to switching of spare circuits. Particularly, the present invention relates to spare circuit control in a cable modem environment.
In order to provide more products to their subscriber base, cable television companies are offering access to the Internet through their cable modem (CM) boxes. The benefits in using the cable companies instead of a dial-up Internet Service Provider is multiple services under one bill, always-on access, and, in some cases, higher speed access.
In order to provide their customer""s with Internet access, the cable companies use some of the 50-800 MHz spectrum typically set aside for their television channels to provide the bandwidth required for the data transfers. A typical cable system has the bandwidth to provide 100 television channels to its subscribers. Each NTSC television signal requires 6 MHz of bandwidth.
In order for a cable subscriber to access the Internet through their cable television provider, the subscriber must have a CM. The CM is similar to the Cable Modem Termination System (CMTS) equipment required at the cable company""s headquarters, except for the greater size required at the headquarters. This is to accommodate a greater number of signals than is required by the home modem.
The home CM box and the CMTS use well-known Ethernet frames to communicate between them. The cable system, however, uses a different modulation scheme, Quadrature Amplitude Modulation (QAM), than is normally used in an Ethernet scheme.
Using QAM, the downstream (from the cable company equipment to the home CM) data rate is in the range of 30-40 Mbps for each 6 MHz channel. This can typically accommodate between 500 and 2000 subscribers. The more subscribers that the cable company tries to fit in that spectrum, however, the data rate provided to each subscriber must be reduced.
The upstream data flow is different and more complex. In the past, cable companies did not have to worry about providing bandwidth for the customer to communicate in the upstream direction. Pay-per-view programming such as movies and sports events, however, required this ability. The cable companies, therefore, set aside the 5-42 MHz spectrum to allow the home CM to communicate in the upstream direction. The cable companies now use this 5-42 MHz spectrum to provide the necessary upstream access to the Internet from the home CM.
Cable companies, as well as other Internet Service Providers, are currently introducing Quality of Service (QoS) to Internet access. The current Internet routing model of xe2x80x9cbest effortxe2x80x9d service now provided to all users, packets, and traffic flows is being replaced with services that differentiate between packets.
FIG. 1 illustrates a typical prior art CMTS block diagram. The CMTS typically is comprised of a cable interface card (101) to provide the interface signals and modulation to the signals transmitted to the home modem. An Ethernet card (110) interfaces the CMTS to the Internet by providing appropriate timing, control, and data signal formats for the Internet. A buffer circuit (105) between the cable interface card (101) and Ethernet card (110) stores data in both the upstream and downstream directions when the processing in either the cable interface card or the Ethernet card is slower than the incoming data.
FIG. 2 illustrates a typical prior art circuit card layout of a CMTS. This CMTS is comprised of a number of cable interface cards (201-205) connected to a mid-plane (210). Each cable interface card (201-205) has an associated switch card (211-215). Each cable interface card is associated with a number of cable customers.
When one of the cable interface cards fails, those customers lose service until the failed card is manually swapped out for a good one. There is a resulting unforeseen need for a more reliable cable interface system to provide an economical back up to the cable interface cards.
The present invention encompasses a cable interface circuit back-up using spare cable interface circuit cards in conjunction with a switching scheme. The system of the present invention is comprised of a spare cable interface card assigned to a predetermined number of cable interface cards. In the preferred embodiment, a first spare cable interface card is assigned to two operating cable interface cards and a second spare cable interface card is assigned to one operating cable interface card.
The system of the present invention provides multiple spare circuit availability to multiple active circuits. The system is comprised of a plurality of active circuit cards, each active circuit card having circuitry that provides an output signal and that accepts an input signal.
The spare circuit card that is assigned to the N active circuit cards is comprised of circuitry that is substantially similar to the circuitry in the N active circuit cards. This spare circuit card acts as a back-up upon failure of one of the assigned active circuit cards.
The system further comprises a plurality of switching cards. Each switching card of the plurality of switching cards is paired with an active circuit card or a spare circuit card. Each switching card that is associated with an active circuit card also comprises an output for outputting a signal and an input for accepting a signal.
A controller circuit is coupled to each of the active circuit cards, each of the spare circuit cards, and each of the switching cards. The controller card provides a switch control signal to the switch card that is paired with a particular active circuit card when the behavior of that particular active circuit card becomes suspicious, indicating a failed active circuit card.